1. Field of the Invention
The present invention relates to a filter regenerating apparatus for an internal combustion engine, and in particular, for regenerating a filter provided to scavenge a particulate included in exhaust gas discharged from an internal combustion engine. More particularly, the present invention relates to a filter regenerating apparatus equipped with an improved detecting device for detecting an amount of the particulate scavenged by the filter utilizing an electric wave.
2. Description of the Prior Art
In the field of the global environmental conservation, how to deal with the global warming and how to reduce the amount of CO.sub.2 in the atmosphere is a matter of the primary importance today. Moreover, how to reduce acid rain which is destroying forests is an equally critical subject.
Acid rain is a natural phenomenon caused by the air polluted by the chemicals such as sulfur oxide and nitrogen oxide. Recently, the controls over discharge of these polluting chemicals have been tightened worldwide both for fixed sources (handled by the cogeneration system) and mobile sources such as vehicles. Special attention has been paid to the exhaust gas from vehicles, which is subjected to density controls. Most notably, it is now being discussed whether to replace the present density controls with amount controls. In addition, the limits are also planned to be greatly lowered.
Among vehicles, the one using an internal combustion engine is the subject of some of the more stringent restrictions. More strict regulations are being considered for exhaust particulate as well as for nitrogen oxides. In the past, the polluting chemicals included in the exhaust gas have been reduced by an improved combustion method of, for example, delaying the fuel injection time. However, such a method is considered to be insufficient to achieve the goal of global environmental conservation.
Therefore, it is required today to attach an after treatment device for the exhaust gas. The after treatment device includes a filter for scavenging the particulate. However, if the particulate is scavenged to the full capacity of the filter, the filter soon becomes clogged. Accordingly, the scavenging capability of the filter is lowered and a smooth flow of the exhaust gas is prevented. As a result, the engine output is significantly reduced or stopped.
Under these circumstances, technological efforts to regenerate the filter have been made worldwide. However, none of these efforts have been put into practical use.
It is known that the particulate is combusted at a temperature of 600.degree. C. or higher. As a medium for generating energy to heat the particulate to such a high temperature, a burner, an electric heater, a microwave and the like have been considered. Whichever the medium is used, an amount of the particulate scavenged by the filter must be detected in order to efficiently regenerate the filter.
A method for detecting the amount of the particulate using an electric wave is disclosed in U.S. Pat. No. 4,477,771. FIG. 6 in the present application shows an apparatus disclosed in the above patent. The apparatus includes an exhaust pipe 1 through which an exhaust gas discharged by the internal combustion engine is flowed, a heating room 2 provided in a portion of the exhaust pipe 1, a filter 3 accommodated in the heating room 2 so as to scavenge exhaust particulate, a microwave generating device 4 for generating a microwave to be supplied to the heating room 2, a microwave supplying antenna 5 for radiating the microwave generated by the microwave generating device 4 in the heating room 2, and a microwave detecting antenna 6 for detecting the energy level of the microwave returned from the heating room 2 through the filter 3. The amount of the particulate scavenged by the filter will be referred to herein simply as the "particulate amount".
In the above construction, as the particulate amount is increased, the effective dielectric factor is changed. Accordingly, a resonant condition of the microwave in the heating room 2 is changed. The change in the resonant condition is monitored based on a change in transmission characteristics of the microwave, and thus the particulate amount is detected.
Such a conventional apparatus still has drawbacks associated with detecting the particulate amount, and with the construction which prevent a highly precise detection of the particulate amount. The biggest problem is attributed to the characteristics of the particulate with respect to the microwave. The main component of the particulate is carbon. The aforementioned patent employs a principle utilizing only static characteristics of the filter. In other words, the particulate amount is detected simply based on a change of the dielectric factor of a space including the filter. As the particulate amount is increased, the effective dielectric factor of the above space is increased. The dielectric loss of the above space is also increased because the particulate absorbs the microwave. Accordingly, the microwave in the heating room 2 is concentrated in the above space. The change of the effective dielectric factor and the dielectric loss of the filter also increases the energy level of the microwave stored in the filter 3.
Since the filter 3 is interposed between the microwave supplying antenna 5 and the microwave detecting antenna 6, the level of a signal detected by the microwave detecting antenna 6 is changed in a complicated manner in accordance with the change of the microwave distribution and the energy level of the microwave stored in the filter 3. Due to the complicated change of the level of the signal, it is difficult to detect the particulate amount with sufficient precision to determine the appropriate timing to heat-combust the particulate for filter regeneration.
Another problem associated with the conventional apparatus is that the microwave detecting antenna 6 is disposed in the heating room 2. With such a construction, it is difficult to provide the microwave detecting antenna 6 and related peripheral devices with sufficient protection from the heat generated for combusting the particulate. Further, the particulate accumulates in the vicinity of the microwave detecting antenna 6, thereby preventing stable detecting precision. As a result, a highly reliable detecting antenna 6 cannot be realized.